2.4 Cell Membranes and Transport

2.4 Cell Membranes and Transport

The cell membrane consists of structures called phospholipids

Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

Phospholipids: a lipid molecule comprised of a polar (hydrophilic) phosphate head and a non-polar(hydrophobic) fatty acid tail.

Hydrophilic: polar, water loving head made of phospholipid.

Hydrophobic: nonpolar, water fearing tails made of 2 fatty acid chains

Cholesterol is a component of animal cell membranes. The cholesterol makes the phospholipid bilayer less

fluid at moderate temperatures, but slightly more liquid than it normally would be at cold temperatures. This helps maintain its functionality and strength over a wide range of temperatures.

Davson-Danielli Model NOTE: this model is not correct but shows the

progression of the current model Lipid bilayer composed of phospholipids

Hydrophobic tails inside Hydrophilic heads outside This forms two separate water-interacting

surfaces Proteins coat outer surface

This forms a protein-lipid sandwich Proteins do not permeate the lipid bilayer

Problems with this model:

This model assumes that all membranes are identical - this was known to be false (composition varies from cell to cell)

The membrane proteins would be exposed to hydrophilic environments on all sides (from the phospholipids and from the water of the cytoplasm). This is not a stable configuration.

Fluid Mosaic Model of Cell membranes

A model conceived by S.J. Singer and Garth Nicolson in 1972 to describe the structural features of biological membranes.

The plasma membrane is described to be fluid because of its lipids and membrane proteins that move laterally or sideways throughout the membrane. That means the membrane is not solid, but more like a 'fluid'.

It is a mosaic because it is made of many different parts like such as integral proteins, peripheral proteins, glycoproteins, phospholipids, glycolipids, and in some cases cholesterol, lipoproteins. Because it is fluid, the membrane's shape may be altered.

http://www.youtube.com/watch?v=Qqsf_UJcfBc

Proteins Found within the Plasma Membrane

Examples of Membrane Proteins:

Membrane proteins are diverse in terms of structure, position in the membrane and function.

1. Transport Channel Proteins (D): Proteins pores (w/ 'lock & key gate') through

which ions and macromolecules may pass.

2. Enzymes (I): Membranes provide a convenient surface for

enzymes to be embedded. Enzymes for related reactions are organized next to each other, organizing the reactions for greater efficiency.

(ex. Electron transport chain)

3. Cell Surface Receptors Many proteins have 'lock & key' surfaces,

which will only fit to specific substances. When the chemical binds to the protein, it is brought into the cell (via facilitated diffusion). This is how some large food macromolecules, chemical messengers, or even viruses enter cells.

responsible for cell-cell signaling Receptors communicate by binding to

neurotransmitters, peptide hormones, and growth factors)

4. Cell Surface Identity Markers (E):

Glycoproteins act as ID markers which identify the cell as belonging to that individual organism. Ex. Antigens – responsible for antibody

response

5. Cell Adhesion Proteins (A): Adjacent cells stick together via

interlocking proteins on their membranes. Ex. Desmosomes-a cell structure specialized

for cell-to-cell adhesion.

Desmosome:

Membrane permeability can be(ability to allow chemicals to pass / not pass across it):

permeable- holes in membrane are large & all molecules pass through.

semi-permeable- smaller holes in membrane allow only small molecules to pass through.

non-permeable - no holes in membrane. Nothing passes through.

http://www.youtube.com/watch?v=y31DlJ6uGgE – Bozeman science

Membrane Permeability and Transport

Examples of Cell Transport

Passive Transport - Movement of molecules from high to low concentration. Needs no energy input.

Active Transport - Movement of molecules from low to high concentration (opposite the flow of diffusion). Needs input of energy (ATP).

Concentration

Active

Passive

Examples of Passive Transport: 1. Diffusion: Random movement of

any molecules from higher to lower concentration.

2. Osmosis: Diffusion of water across a membrane. Occurs from high concentration of H2O to low conc. H2O

http://www.youtube.com/watch?v=w3_8FSrqc-I

Terms Associated with Osmosis in Cells: Hypertonic: (hyper = more /

tonic = solute)

a) Any solution having more solute (less H2O) than inside cell. More water flows out of cell so cell shrinks.

Can shrivel the cell - crenation

Terms Associated with Osmosis in Cells:

Hypotonic: (hypo = less; tonic = solute)

a) Any solution having less solute (more H2O) than the cell contents. Water diffuses into the cell (which has more solute, less H2O) so cell expands.

Can burst the cell -hemolysis

Terms Associated with Osmosis in Cells:

Isotonic: (iso = equal/tonic = solute)

2 solutions with equal concentrations of solute and water. Net flow of water between the 2 is equal. We say both are in equilibrium. H2O diffusion still occurs in both directions at equilibrium

3. Facilitated Diffusion: passive transport (no energy required).

Passage of molecules from high to low concentration (diffusion) across membrane via binding to a membrane protein which carries it across.

Animation: [Facillitated Diffusion]

Facilitated Diffusion:

Ion Channels

Facilitated diffusion with Ion channels: Special channel proteins allow ions to pass through membrane w/out interacting with hydrophobiclipid tails from high to low concentration.

Active Tansport

Active Transport: requires large amounts of energy (ATP) to 'pump' materials against force of diffusion (low to high).

Includes endocytosis (phagocytosis, receptor mediated endocytosis and pinocytosis); exocytosis; ion channels (low to high concentration)

The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis. Vesicles move materials within cells.

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio02.swf::Endocytosis%20and%20Exocytosis

Types of Endocytosis: 1) Phagocytosis: Endocytosis (bulk passage):

move large polar molecules. This action requires energy input (ATP) by cell.

2) Pinocytosis: cell drinking, liquid material brought into cell by vacuole formation.

Animations: [Pinocytosis] [Pinocytosis]

B. Exocytosis: materials discharged from cell (vacuole passes plasma membrane to dump contents out of cell).

http://www.youtube.com/watch?v=U9pvm_4-bHg

C. Sodium-potassium pump. Moves 3 Na + out of cell & 2 K+ into cell. Both Na+ and K+ are moved from low to high concentration.

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Documentary https://www.youtube.com/watch?

v=FFrKN7hJm64&list=PL5BgsIwbuBEjQn8IbS4gyfhTiaji__gc2 –BBC Secret Universe